QCX assembly Rev 1.08
112
5.9
Transmit signal routing and PA driver
The 74ACT00 is a quad NAND logic gate. The
input threshold voltage for a binary “1” is 2.4V
which means that the gate is easily switched on by
the ~3.3V peak-peak squarewave output from the
Si5351A. The output of the 74ACT00 is 5V peak-
peak, perfect for driving the BS170 MOSGETs in
the Class-E PA into saturation.
The Clk2 signal from the Si5351A is used as the
transmit oscillator as previously mentioned. It
would have been easy to enable/disable the Clk2
output in software in the Si5351A chip
configuration. However, this transceiver design also includes the built-in signal generator feature,
for aligning the Band Pass Filter and adjusting the I-Q balance and phase adjustment controls. So,
some of the spare gates in the 74ACT00 are used to switch the signal generator on/off and the RF
Power Amplfier (PA) signal on/off, separately.
When the SIG OUT control line from the microcontroller is high, the Clk2 signal is enabled as
signal generator, and routed via a 120K resistor straight to the RF input of the transceiver.
When the TX control line from the microcontroller is high, the Clk2 signal is routed to the PA. The
final inverting gate IC3A is added to make sure that when the TX gate IC3B is off, the driver
voltage presented to the BS170 gate is low, so the transistors are off.
5.10
Class-E Power Amplifier
A Class-E power amplifier is a wonderful
thing. It has a very high efficiency,
sometimes over 90%. This has several
important benefits:
a) Since not much power is
dissipated, we can use smaller
(and cheaper) transistors
b) So little power is wasted as heat
that the requirement for a
heatsink is reduced or eliminated
c) During transmit the radio requires
less current, so the drain on a
battery is less
– important for
people who want to operate
portable.
A Class-E Power Amplifier contains a
resonant circuit at the frequency of operation, so it is only suitable for single-band use. A lot has
been written about Class-E, much of it is very technical and mathematical.
Summary of Contents for QCX 5W CW
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